A pierce-type electron gun has a characteristic of being able to separate a beam generation source and a vacuum atmosphere including an irradiation target and able to maintain a stable beam generation source. Further, the pierce-type electron gun is widely used as a heating source for a vapor deposition apparatus, a melting furnace and a heat-treating furnace because the pierce-type electron gun uses electrons as an energy source, and is capable of easily scanning and deflecting. The application field of the pierce-type electron gun is expected to become broader and broader in the future, especially as a heating source of an in-line vapor deposition apparatus for metal oxide such as MgO and SiO2 in which long-time stability over 300 hours or more is required, as a heating source of a roll-to-roll vapor deposition apparatus that is capable of heating to a predetermined evaporation rate in a short time, in which it is required to evaporate, stably and at a large capacity, metal such as Al, Co—Ni and Cu in an excellent deposition thickness distribution, as a heating source of a roll-to-roll vapor deposition apparatus for optical layers in which metal oxide layers of SiO2, ZrO and the like are reproducibly evaporated in a deposition thickness distribution within ±1%, and the like (examples of use of the electron gun in the MgO vapor deposition apparatus and the roll-to-roll vapor deposition apparatus are shown in FIGS. 9 and 10, respectively).
Especially, in recent years, in order to deal with trends of increase in size of a mother glass and commercial production, there has been a demand for more homogenous and high-speed deposition of a magnesium oxide (MgO) layer which is used as a protective layer for a surface of a grass substrate for a plasma display panel (PDP).
Because of such background, an electron beam vapor deposition apparatus including a plurality of pierce-type electron guns (FIG. 15) has been developed, for example. This apparatus basically has a structure as shown as electron beam apparatuses 81, 82, in each of which two chambers of a loading/unloading chamber and a vapor deposition chamber or three chambers of a loading chamber, a vapor deposition chamber, and a unloading chamber are connected via a gate valve.
The outline of the vapor deposition chamber 2 in the electron beam vapor deposition apparatuses 81, 82 is as shown in FIG. 9. That is, as a heating source for continuously forming MgO as a protective layer for the PDP, a pierce-type electron gun 3 is mainly used. An electron beam F approximately horizontally emitted from the pierce-type electron gun that is fixed to a side wall of the vapor deposition chamber is deflected by an electron beam deflection device 20 and focused on an evaporation point P of MgO 11 in a hearth 4 as an evaporation source, to thereby generate an MgO vapor flow and form an MgO layer on a glass substrate 10 mounted on a carrier that passes and moves thereabove. That is, the vapor deposition chamber doubles as an irradiation chamber of the electron beam.
The electron beam vapor deposition apparatuses 81, 82 have features that the vapor deposition chamber is prevented from being exposed to the atmosphere, and an atmosphere in the vapor deposition chamber can be stably maintained because a pretreatment such as degassing and heating treatments can be performed with respect to the glass substrate 10 or a carrier mounted with the glass substrate in the loading/unloading chamber 83 or the loading chamber 84, and that the production volume is large as compared with a batch-type apparatus.
However, there is a desire for a stable operation of the pierce-type electron gun for a long period of time.
In this regard, from the past, there has been made various kinds of efforts for the stable operation of the pierce-type electron gun for a long period of time.
For example, there is a case where moisture, residual gas, evaporating particles and the like exist within the vapor deposition chamber, and thermal electrons which constitute the electron beam collide therewith to thereby generate ions, and the ions flow back to cause an abnormal electrical discharge of the electron gun. Thus, a cathode is provided with a through-hole and an ion collector for receiving ions and components that are dispersed by the collision of the ions (for example, see Patent Documents 1, 2).
However, a diameter of the electron beam at an internal of the electron gun and a diameter (power density) of the electron beam which is irradiated on a target fluctuate depending on pressure at the internal of the electron gun and pressure at an atmosphere in which the irradiation target is placed, because of a space charge effect that increases the beam diameter and an energy width due to an interaction between particles which is caused by charges included in electrons, and because of a space charge neutralizing action due to ionization of an atmosphere gas caused by collision of the electrons with the gas. Accordingly, when taking vapor deposition as an example, there has been a problem that a vapor deposition rate lacks stability, and the like. Therefore, there has been a case where a stable operation in a wide area by separating a beam generation source and an atmosphere including an irradiation target, which is one of characteristics of the pierce-type electron gun, cannot be fully utilized.
Moreover, expansion of the beam at the internal of the electron gun may affect and overheat components within the electron gun. As a result, there has been a case where the electron gun itself is damaged.
In this regard, in order to stabilize the beam diameter at the internal of the electron gun, that is, to prevent the beam from expanding widely and avoid the electron gun from being damaged, means have been used such as introducing Ar into the electron gun as a space charge neutralizing gas, adjusting a conductance of a flow register, and providing multiple stages of focusing coils.
Further, an electron beam emitter portion (beam generation portion, generation portion) is stabilized with respect to assembling accuracy and change according to time. In other words, the electron gun itself is optimumly designed such that an angle of a cathode surface, an angle of a Wehnelt, an angle of an anode, a gap between the cathode and the Wehnelt, a gap between the cathode and anode, and the like comply with the above object. This is performed with the aim of stabilizing a beam focusing condition that depends on an electric field.
However, in both cases, there is no appropriate feedback means, and the electron gun is operated with preset values. Thus, it has been difficult to perform a stable and accurate deposition process. Further, an inert gas such as Ar may also affect the deposition process.
In this regard, a method has been proposed in which a beam diameter is measured at a beam outlet and a beam irradiation portion and fed back to a beam current and a focusing coil current (see Patent Document 3). As shown in FIG. 14, monitor pieces XR 1, XR 2, XR 3, XL 1, XL 2, XL 3, which are capable of outputting a beam point temperature of the electron beam as an electric signal, are provided in a vicinity of a ring hearth 4 as the beam irradiation portion to feedback the beam point temperature to the beam current and the focusing coil current to thereby achieve stabilization.
However, there remains an influence of the space charge effect in the internal of the electron gun. Thus, it is less than perfect.
Patent Document 1: Japanese Patent Application Laid-open No. 2004-14226 (page 3, FIG. 1)
Patent Document 2: Japanese Patent Application Laid-open No. 2005-268177 (page 3, FIG. 1)
Patent Document 3: Japanese Patent Application Laid-open No. 2005-264204 (page 3, FIG. 1)